i) Field of the Invention:
The present invention relates to an electric vehicle provided with a hydraulic brake means and a regenerative brake means, and more particularly to a brake control system provided with a means for improving braking feeling and a means for performing a hydraulic control depending on a regenerative characteristic.
ii) Description of the Related Arts:
An electric vehicle is a vehicle having a motor as a driving source. As a braking system for an electric vehicle, for example, a liquid-pressure brake such as a hydraulic brake or the like and a regenerative brake using primary current control or the like of a motor are known.
The hydraulic brake is a braking means widely used for other kinds of vehicles. In this case, an oil pressure is generated depending on the amount of depression of a brake pedal and the generated oil pressure is transmitted to wheel cylinders or the like via piping to brake wheels of a vehicle. Since the wheels are mechanically braked by using the transmitted oil pressure, this brake can be used for both drive wheels and idle wheels.
The regenerative brake uses a principle of a regeneration of a motor for running. Hence, the regenerative brake is only mounted on vehicles such as a usual electric vehicle and a hybrid vehicle which also has an engine mounted. For example, when an induction motor as a running motor is used, by controlling a primary current of this induction motor, a necessary output torque can be obtained. This control, for instance, is carried out as a control of the primary current vector by a PWM (pulse width modulation) control of an inverter circuit. The regenerative brake can be realized by a part of this torque control, that is, the primary current control is carried out so that the motor may be operated as a generator so as to obtain a regenerative torque. Thus, the regenerative brake is used solely for braking the drive wheels.
When the hydraulic brake and the regenerative brake are used together, care must be taken. That is, it is necessary to adjust a brake force distribution between the drive wheels and the idle wheels so as to obtain a required braking force. In the electric vehicle or the hybrid vehicle, a rechargeable battery as a power source (at least a part thereof) of the running motor is used. Hence, by properly performing the brake force distribution, the regeneration of the motor is effectively carried out, and as a result, the possible running distance per unit battery charge can be elongated.
In a conventional vehicle braking system, for example, as disclosed in Japanese Utility Model Laid-Open No. Sho 63-29301 front drive wheels are braked by a regenerative brake and rear idle wheels are braked by a hydraulic brake. In this case, an oil pressure is changed so as to optimize a brake force distribution between the front and rear wheels. Hence, in this system, by properly performing the brake force distribution between the front and rear wheels, an electric vehicle can be smoothly braked. Further, the regeneration of the motor can be effectively carried out and the possible running distance per unit battery charge can be elongated.
In another conventional vehicle braking system, for instance, as disclosed in Japanese Patent Laid-Open No. Sho 64-43001, a hydraulic brake force is controlled to a value obtained by subtracting a regenerative brake force from a necessary brake force. In this case, the energy recovery due to the regeneration of the running motor can be suitably performed, and the possible running distance per unit battery charge can be elongated.
However, in the conventional vehicle braking system, when the brake force control is performed by the oil pressure change, a new problem arises. That is, when the brake force control is executed by the oil pressure change, the optimization of the brake force distribution is possible and an early locking of the wheels such as the front wheels braked by the regenerative force can be prevented. However, on the contrary, the stroke of the brake pedal is changed, and thus the braking feeling becomes bad.
Further, in the conventional vehicle braking system, when the front wheels are braked by the hydraulic brake and the regenerative brake, another problem occurs concerning a characteristic of an induction motor used for a running motor, that is, the maximum regenerative brake force (the maximum regenerative torque) is reduced in a high range of a revolution rate of the motor. That is, the maximum regenerative brake force at the high speed revolution of the motor (see FIG. 5A) becomes small compared with the maximum regenerative brake force at the low speed revolution of the motor (see FIG. 5B). Accordingly, the total brake force (the sum of the regenerative brake force and the hydraulic brake force) also becomes also small. Also, at the time of low speed revolution, when the maximum regenerative brake force is large, even when the necessary brake force is lower than the regenerative brake force, the hydraulic brake force is operated and thus an excessive brake force is often applied.